attn: starfish larva bilaterally symmetric

[eub]

http://www.microscopy-uk.org.uk/mag/art98/janstar.html

The common starfish develops a so-called bipinnaria larva, with ciliated bands running about the periphery, a beautiful sight.

After several weeks the bipinnaria larva takes on a more elaborate form, with longer projecting arms and after some more weeks, a brachiolaria larva is formed. The larvae have their own gut, with inside cilia to inhale and transport food particles. They feed themselves with diatoms and other organisms in the plankton. The stomach is large and round and situated at the back side.

After this phase a large part of the larva degenerates and at the rear side a rudimentary formed juvenile starfish develops. The organs of the young starfish are formed anew.

Comments

[jinian.livejournal.com]

Deuterostome!

[hosterman.livejournal.com]

Cool!

[eub.livejournal.com]

Do you know if the the starfish's evolutionary history is known, like were its ancestors bilaterally symmetric adults right up until they added a "form a radially symmetric outgrowth and throw the rest away" ontogeny step? (And how do they coordinate cells into that symmetry? How do they /do/ that with their homeobox?)

[eub.livejournal.com]

http://www.palaeos.com/Invertebrates/Pieces/Hox/Hox.3.html

The Echinoidea (sea urchins) appear to be unique in having completely rearranged the linear relationships of the hox system. In the urchin Strongylocentrotus, the anterior class hox genes and hox3 are found (a) downstream of the posterior class and (b) in reverse order. Richardson et al. (2005) [7]. As you might expect, the hox1, 2, and 3 homologues are oriented backward -- but so also are hox5 and hox11/13b. The urchin evx gene is still the downstream bookend to the system, but now located next to hox1, instead of the most posterior hox gene (here, hox11/13c).

But what does "posterior" mean to a radially symmetrical sea urchin? The answer is complex. Only two hox genes are expressed during urchin embryonic development, hox7 and hox11/13b. However, echinoid development is indirect. The sequence hox7, 8, 8/10, 11/13a, 11/13b is used, in an anatomically collinear way, and from mouth to anus, as the larva curls around the pentameric adult rudiment to form the radially symmetrical adult body.

filter house

[eub.livejournal.com]

The hox clusters of urochordates are often said to have "disintegrated." Varying numbers of middle and/or posterior class hox genes are missing, the various hox genes have gone their separate ways, and genetic colinearity tends to be lost. These trends are taken to extremes in the larvacean Oikopleura, which has only one remaining middle class hox and no contiguous hox genes. However the Larvacea tend to be extreme minimalists, often dispensing with whole organ systems (heart, gills, etc.) and great swathes of DNA, in a profligate manner entirely unbecoming of a chordate. Swalla et al. (2000).